Heattreatment of steel

ABSTRACT

The present invention relates to the heattreatment of steel. More particularly it relates to the heattreatment of all types of iron and steel which are capable of hardening on heating to the austenitising range and then quenching, without the need to introduce additional carbon during the heating step. For the sake of simplicity these types of iron and steel will be referred to hereinafter as &#39;&#39;&#39;&#39;steel of the class which can be hardened by heating and quenching.&#39;&#39;&#39;&#39; This class of materials comprises the plain carbon steels and cast steel of the medium- and high-carbon varieties and the alloy steels containing at least 0.25 percent by weight carbon. The present invention provides a method of heattreating steel of this class whereby the steel is provided with a surface layer that is resistant to scuffing and wear when subjected to friction under a heavy load and the underlying core is hardened so as to be resistant to indentation.

United States Patent [7 2] Inventor Frederick David WaterfallBirmingham, England [21] Appl. No. 722,573

[22] Filed Apr. 19,1968

[45] Patented Oct. 26, 1971 [73] Assignee Imperial Chemical IndustriesLimited London, England [32] Priority Apr. 28, 1967 [33] Great Britain[54] HEATTREATMENT OF STEEL 5 Claims, No Drawings [52] U.S. Cl 148/15.5,148/15 [51] Int. Cl C23c 9/16 [50] Field of Search l48/l2.l, 15, 15.5,16.5, 16,6, 134

[56] References Cited UNlT ED STATES PATENTS 1,499,801 7/1924 Bond148/15.5

2,264,431 12/1941 Bradley 148/15 2,339,223 l/1944 Holt l48/l5.5

2,927,875 3/1960 Mueller l48/l5.5

3,194,696 7/1965 Mueller.... l48/l5.5 X

3,158,514 11/1964 Saxton 148/16.6X

3,228,807 l/l966 Mitchell 148/16.6X FOREIGN PATENTS 1,052,668 12/1966GreatBritain 148/15.5 OTHER REFERENCES Anslegeschrift-Germany, 1170217,May 14, 1964 Metals Handbook, 8 th Ed., Vol. 2, 1964, pub. by ASM, pages129- 132 Primary Examiner-Charles N. Lovell Attorney-Cushman, Darby &Cushman ABSTRACT: The present invention relates to the heattreatment ofsteel. More particularly it relates to the heattreatment of all types ofiron and steel which are capable of hardening on heating to theaustenitising range and then quenching, without the need to introduceadditional carbon during the heating step. For the sake of simplicitythese types of iron and steel will be referred to hereinafter as steelof the class which can be hardened by heating and quenching." This classof materi als comprises the plain carbon steels and cast steel of themediumand high-carbon varieties and the alloy steels containing at least0.25 percent by weight carbon. The present invention provides a methodof heattreating steel of this class whereby the steel is provided with asurface layer that is resistant to scuffing and wear when subjected tofriction under a heavy load and the underlying core is hardened so as tobe resistant to indentation.

According to the present invention we provide a method for the heattreatment of steel of the class which can be hardened by heating andquenching, which comprises heating the steel into its austenitizingtemperature range in a heating medium which is noncarburizing towardsthe steel, then soaking the hot steel in a molten salt bath held at atemperature in the range 700770 C. and containing at least percent byweight and preferably at least percent by weight of alkali metal cyanatecalculated as NaCNO for at least 3 minutes and in any case where thetemperature of the salt bath is in the range 700-750 C. preferably forat least 5 minutes, and then rapidly cooling the steel.

The austenitizing step may be carried out in a nonoxidizing gasatmosphere or in a neutral salt bath as known in the art. Alternatively,for high-carbon steels a conventional gas carburizing atmosphere or asalt bath containing cyanide salt may be employed. Although these mediaare generally known as carburizing media they are essentiallynoncarburizing in action towards high-carbon steels.

For the second stepof the method according to the invention we prefer toemploy a salt bath comprising alkali metal cyanide, alkali metalcyanate, alkali metal carbonate and optionally alkali metal chloride, asdescribed in British Pat. specification Ser. No. 1,052,668 in connectionwith a socalled carbonitriding process. When the cyanate content of thebath is to be maintained at the preferred minimum of 10 percent byweight or higher, this can be achieved in the manner taught in the saidBritish Specification but since the treatment temperatures used in thepresent invention are in general higher than the carbonitridingtemperatures taught in the said British specification the iron contentof the alloy steel surfaces of the container and any electrodes incontact with the molten salt bath should not exceed 22 percent by weightand should preferably be between 10 percent and percent by weight. Atreatment time of about 10 minutes in this type of bath is generallymost useful and we have found that this treatment creates a compoundlayer rich in nitrogen and carbon which can be observed at the surfaceof an etched microsection of the treated steel. This layer at theimmediate surface of the steel varies in depth from about 0.0003 inch to0.002 inch, depending on the cyanate content and temperature of thebath, and there may also be some penetration of nitrogen further intothe steel. During this treatment the steel beneath the surface layerremains mainly or wholly in the austenitic state.

After treatment in the salt bath of the second step the steel is rapidlycooled, for instance by quenching in water, oil or any otherconventional quenching medium which is suitable for hardening of theparticular steel. After quenching it is found that while the underlyingsteel or iron is hardened, with the hardest zone near the surface, theoutermost layer rich in carbon and nitrogen about 00003-0002 inch thickremains unchanged and possesses a very high resistance to scuffing andwear.

An alternative procedure where the usual austentitizing temperature ofthe steel lies within the temperature range 750-770 C. is to carry outthe austenitizing and to apply the carbon-nitrogen compound layer in thesame bath. To do this the parts are heated for austenitizing by placingthem directly into the said salt bath containing at least 5 percent byweight,.

preferably 10 percent by weight, alkali metal cyanate calculated asNaCNO, soaked for at least 7 minutes after attaining bath temperatureand then quenched to harden the steel.

The combination of a hardened steel or iron and a scuffingresistantsurface renders the method of the invention very useful for applicationto engine valve rockers in both steel and nodular iron, to various typesof transmission gears and to forming tools made from plain carbon oralloy tool steels.

For articles that have to withstand extremely severe frictionalconditions in use, it is preferred to maintain at least 10 percent byweight cyanate calculated as NaCNO in the salt bath used in the secondtreatment step, since this confers maximum resistance to scuffing on thesurface of the treated article. Cyanate contents in the range below 10percent but not less than 5 percent by weight are however quiteacceptable for articles working under less arduous conditions; in factthe scuffing resistance achieved with cyanate contents in this range issurprisingly high and well within many commercial requirements.

The invention is further illustrated by the following example, in whichall percentages are by weight, but is not limited thereby.

EXAMPLE l This example shows for purposes of comparison the resultsobtained with a conventional hardening treatment of a tool steel.

Pieces of 0.9 percent carbon tool steel in the shape of discs 0.4 cm. indiameter suitable for testing on an Amsler weartesting machine werehardened in the conventional manner by heating for 10 minutes in aneutral slat bath consisting of a mixture of the chlorides of sodium andpotassium at 780 C. and then quenching in water. Hardness tests showedRockwell Hardness C65-66 on the treated discs. Two treated discs, A andB, were then submitted to a wear test on an Amsler weartesting machine.In this test the discs were rotated in edge-toedge contact under a loadof 20 kilograms at 400 and 440 r.p.m. respectively so that at the pointof contact they moved in opposite directions, and they were lubricatedwith SAE 30 mineral oil. The discs were removed and weighed after L000,9,000 and 25,000 revolutions. The test was stopped at 25,000 revolutionssince by this time the discs were badly scuffed.

The loss in weight of the discs was then as follows:

0.4 cm. diameter Amsler wear test discs made in 0.9 percent carbon toolsteel, as used in example I, were treated according to the method of theinvention as follows. The discs were austenitized by heating for 10minutes at 800 C. in a neutral salt bath consisting of a mixture of thechlorides of sodium and potassium and were than transferred directly toa salt bath maintained at 730 C. and containing 26.1 percent NaCN andl3.7 percent NaCNO, the balance being alkali metal carbonate. The discswere held in this bath for 10 minutes and were then quenched in water.

Examination of the treated discs showed the following hardnesscharacteristics:

Vicken Hardness HVZh 445-463 Viclrers Hardness V30 413-4 l 8 RockwellHardness RHC 38-40 Two treated discs, A and B, were submitted to a weartest on an Amsler machine as in example I. The discs were removed atintervals and weighed. They showed no scuffing when the test was finallystopped after a total of one million revolutions. The loss in weight ofthe discs was then:

Grams Disc A 0.0056 Disc B 0.0048

Total Loss 0.0l04

EXAMPLE 3 Victers Hardness "V2 is 445454 Vickers Hardness "V30 680-690Rockwell Hardness HRC 61-63 Two treated discs, A and B, submitted towear testing on an Amsler machine as in example 2 showed no scuffingafter a total of one million revolutions and the loss in weight of thediscs during the test was as follows:

Grams Disc A 0.3256 Disc 5 0.2430 Total Lou 0.5686

What we claim is:

l. A method for the heat treatment of steel of the class which can behardened by heating and quenching, which comprises heating the steelinto its austenitizing range in a heating medium which is noncarburizingtowards the steel for a time sufficient to heat the steel throughout.then transferring the hot steel to a molten salt bath held at atemperature in the range 700-770 C. and containing at least 5 percent byweight of alkali metal cyanate calculated as N CNO and a remainderconsisting essentially of alkali metal cyanide together with a saltselected from the group consisting of alkali metal carbonate andmixtures thereof with alkali metal chloride, and soaking the steel insaid salt bath for at least 3 minutes and then rapidly cooling thesteel.

2. A method according to claim I, wherein the said molten salt bathcontains at least 10 percent by weight of alkali metal cyanatecalculated as NaCNO.

3. A method according to claim 1, wherein the step of heating the steelinto its austenitizing range is carried out by immersing the steel in aneutral salt bath.

4. A method according to claim 1, wherein the steel is a high-carbonsteel and the step of heating the steel into its austenitizing range iscarried out by immersing the steel in a molten salt bath containingalkali metal cyanide.

5. A method according to claim 1, wherein the said molten salt bath heldat a temperature in the range 700 770 C. is restricted to the range700-750 C. the hot steel is immersed in this bath and is soaked thereinfor at least 5 minutes.

2. A method according to claim 1, wherein the said molten salt bathcontains at least 10 percent by weight of alkali metal cyanatecalculated as NaCNO.
 3. A method according to claim 1, wherein the stepof heating the steel into its austenitizing range is carried out byimmersing the steel in a neutral salt bath.
 4. A method according toclaim 1, wherein the steel is a high-carbon steel and the step ofheating the steel into its austenitizing range is carried out byimmersing the steel in a molten salt bath containing alkali metalcyanide.
 5. A method according to claim 1, Wherein the said molten saltbath held at a temperature in the range 700*-770* C. is restricted tothe range 700*-750* C., the hot steel is immersed in this bath and issoaked therein for at least 5 minutes.